fix(spanner): fix off-by-one in the streaming-query retry attempt count

[fornwall]

Fix attempt count off-by-one in retries for streaming reads, due to attempt and retry count mixup.

See the doc on RetryPolicy::with_attempt_limit:

    /// This policy decorates an inner policy and limits the total number of
    /// attempts. Note that `on_error()` is not called before the initial
    /// (non-retry) attempt. Therefore, setting the maximum number of attempts
    /// to 0 or 1 results in no retry attempts.

Include also a related minor change to use a constant for default number of attempts instead of hardcoding the attempt limit in the test.

[gemini-code-assist]

Code Review

This pull request introduces a DEFAULT_ATTEMPT_LIMIT constant to replace hardcoded retry limits in ResultSet. It also corrects the retry attempt count calculation by including the initial attempt (1 + self.retry_count) and updates the corresponding unit tests to dynamically calculate expected backoffs based on the new constant. There are no review comments, and I have no feedback to provide.